Constable first achieved success (and recognition by the Royal Academy) with his large canvases depicting the Stour Valley, which he exhibited between 1819 and 1825. Working on a scale usually reserved for History painting, Constable redefined the notion of a ‘finished’ picture by giving his large paintings something of the spontaneous freedom and expressive handling of a rapidly painted sketch.

During the 1820s Constable was repeatedly occupied with the motif of the Lock – it could be regarded as his favourite subject. In 1824  he exhibited the fifth in his series of six large Stour Valley paintings at the Royal Academy, ‘A boat passing a lock’, which he subsequently called The lock (Carmen Thyssen-Bornemisza Collection on loan to the Museo Thyssen-Bornemisza, Madrid). It differed from the previous four large canvases in having a vertical format. Constable made at least two other upright versions of the subject in 1824 (Philadelphia Museum and Art Gallery, and private collection). Then, in this painting, he converted  the vertical composition into a horizontal one, extending the scene to the right and varying the action.

Here a boat with a sail on its way up the River Stour waits at Flatford Lock. The boat is tied to a post while the lock keeper opens the gates to allow it to enter the lock chamber, to be lifted to the higher water level before continuing its journey up river. Constable created an open composition, with Flatford Bridge and a further lock gate and a barge in the background on the right. He depicted a heavy rainstorm on the left, and included a dog in the foreground at the right.

The composition was based on two drawings with a horizontal format, Flatford Lock 1823   and Flatford Lock c.1826  . Constable took the rainstorm from an oil sketch of 1819, Branch Hill Pond, Hampstead (Victoria and Albert Museum, London), which herepeated with variations on several occasions, including Branch Hill Pond, Hampstead Heath, with  a boy sitting on a bank c.1825–28  .

Sarah Cove, who has undertaken a detailed technical examination of this picture, ‘discovered via X-ray that the arms of the lock keeper  were originally raised, as in every previous version of the lock keeper’ (Sarah Cove to Anne Lyles, 12 September 2005, NGA file 04/0501–04).

The painting was commissioned in 1826 by the Bond Street picture dealer, print and book publisher, James Carpenter. While working on the commission Constable wrote to Carpenter: ‘I have been at the picture ever since I saw you & it is now all over wet – I was at work on it at  7 o clock this morning – and I should have been at it still’. He added: ‘I wish your picture was as good as Claude Lorraine’ (Beckett IV,  p. 138). Two years after painting this work Constable borrowed it back from Carpenter and re-worked it. He then exhibited this painting at the British Institution in 1829 under the title ‘Landscape and Lock’.

When he was elected a full member of the Royal Academy in 1829 Constable was expected to present a work to the Academy. Such was the value he placed on this painting that he took it back from Carpenter and presented it to the Academy, depositing 100 guineas with a banker until he compensated Carpenter with a work of the same size.